Conventionally, in order to evaluate blood perfusion in the capillaries of myocardium, data which quantitatively represents the perfusion dynamics of myocardium (myocardium perfusion data) are computed using an angiographic medical image captured by a X-ray computed tomography (CT) apparatus, a X-ray diagnostic apparatus, a magnetic resonance imaging (MRI) apparatus, or the like. In addition, the map image obtained by mapping the myocardium perfusion data on the cardiac tissue is created as the myocardium perfusion image.
In general, in order to evaluate the perfusion dynamics of the myocardium, two kinds of examination including rest examination and stress examination are performed. First, in the rest examination, the X-ray CT image is captured, for example, after a contrast agent is injected to a subject at rest. Here, the X-ray CT imaging includes preliminary imaging and actual imaging. The preliminary imaging is carried out to determine whether or not a certain amount of contrast agent arrives at the cardiac. In the preliminary imaging, a lower dose of the X-ray is irradiated than the actual imaging, and the X-ray CT images including interest areas (Region of Interest (ROI)) are sequentially reconstructed by the X-ray CT apparatus. In addition, when a pixel value (CT value; unit: [HU]) in the ROI of the X-ray CT image reconstructed by the preliminary imaging is higher than a predetermined threshold value, the X-ray CT apparatus determines that a certain amount of the contrast agent has arrived at the cardiac. In addition, the X-ray CT apparatus carries out the actual imaging to reconstruct volume data which contains data on the entire cardiac by scanning the entire cardiac, for example, by helical scanning with an increased X-ray dose. Moreover, the rest myocardium perfusion data is computed from a plurality of rest X-ray CT images sequentially captured in the preliminary imaging. Alternatively, the rest myocardium perfusion data is computed from a plurality of rest X-ray CT images sequentially captured after the preliminary imaging.
Then, in the stress examination, the X-ray CT image is captured, for example, after the contrast agent is injected again to the subject who has been subjected to an exercise stress or a drug stress. Then, the stress myocardium perfusion data is computed from a plurality of stress X-ray CT images sequentially captured in the preliminary imaging. Alternatively, the stress myocardium perfusion data is computed from a plurality of stress X-ray CT images sequentially captured after the preliminary imaging. Then, a doctor diagnoses about presence of a disease in myocardium or about a degree of the disease in the subject by comparing the rest myocardium perfusion data with the stress myocardium perfusion data.
However, according to the examination method of the conventional art, the contrast agent is injected twice. Here, for example, if the rest examination and the stress examination are performed at intervals of about an hour, the contrast agent injected in the rest examination is excreted so that the stress examination can be carried out under the same condition as the rest examination. However, in practice, waiting with a needle being inserted until the contrast agent, which is injected in the rest examination, is excreted would put the subject under stress and degrades examination efficiency.
For this reason, in the medical field, if the rest examination is finished, it is demanded that the stress examination is performed as soon as possible. However, the method of computing the myocardium perfusion data in the X-ray CT image or the like is based on the premise that there is no contrast agent in the cardiac of the subject before examination. Therefore, if the stress examination is performed immediately after the rest examination is finished, the contrast agent which has been injected in the rest examination is not excreted but stays in the myocardium or in the blood so that an error may be included in the myocardium perfusion data.